The present invention relates to a hydraulic shift control system for an auxiliary gearing of an automatic transmission, and more particularly to a safeguard of such a shift control.
JP 59-113351 A discloses an automatic transmission including an auxiliary gearing drivingly connected in series to a main gearing.
In this known automatic transmission, the auxiliary gearing is shiftable between a high gear position and a low gear position. It includes a direct clutch as a first frictional device to be engaged in the high gear position, and a reduction brake as a second frictional device to be engaged in the low gear position. The high gear position is established in the auxiliary gearing when the direct clutch is engaged with the reduction brake released, while the low gear position is established in the auxiliary gearing when the direct clutch is released with the reduction brake engaged. The auxiliary gearing provides a direct drive in the high gear position, while it provides a reduction gear in the low gear position. The auxiliary gearing also includes a one-way clutch which becomes operative to serve as a substitute for the reduction brake during engine drive forward running. During a shift in the auxiliary gearing from the low to high gear position by disengaging or releasing the reduction brake and engaging the direct clutch, the one-way clutch anchors a rotary member which is to be gripped by the direct clutch. This prevents excessive wear of the direct clutch and reduces a shock taking place during engagement of the direct clutch.
For reverse drive, the main gearing is conditioned in a reverse gear position, while the auxiliary gearing is conditioned in the low gear position since if the auxiliary gearing is conditioned in the high gear position with the direct clutch engaged and the reduction brake released, the auxiliary gearing which tends to rotate as a unit owing to the engagement of the direct clutch is held stationary by the one-way clutch complementary to the reduction brake. Thus, in order to protect the automatic transmission against damage, the auxiliary gearing must be conditioned in the low gear position with the reduction brake engaged and the direct clutch released.
According to JP 59-113351 A, the auxiliary gearing is shiftable between the high gear position and low gear position by a 3-4 shift valve. The 3-4 shift valve has a high gear position wherein a servo activating hydraulic pressure generated by a regulator valve is supplied to the direct clutch, namely the first frictional device, to engage same and the reduction brake, namely the second fictional device, is drained. It also has a low gear position wherein the direct clutch is disengaged and the reduction brake is engaged. The 3-4 shift valve is shiftable between the high and low gear positions in response to a solenoid hydraulic pressure generated by an electro-hydraulic transducer, a spring and a hydraulic pressure signal supplied thereto via a predetermined hydraulic fluid line. The electro-hydraulic transducer is of the on drain type which generates the solenoid hydraulic pressure when its solenoid is deenergized. The solenoid hydraulic pressure urges the shift valve toward the high gear position thereof against the action of the spring. A 2-3 shift valve operatively associated with another electro-hydraulic transducer is provided. This electro-hydraulic transducer is of the on drain type which generates a solenoid hydraulic pressure when its solenoid is deenergized. This solenoid hydraulic pressure urges the 2-3 shift solenoid against a spring toward a position wherein the predetermined fluid line is drained when the automatic transmission has a forward drive range. In response to energization of the solenoid of the electro-hydraulic transducer for the 2-3 shift valve, the above-mentioned fluid line is allowed to communicate with the pressure regulator to receive the servo activating hydraulic pressure when the automatic transmission is conditioned in the first or second gear position during this drive range or when the automatic transmission is conditioned for a reverse drive range. The predetermined fluid line is drained when the automatic transmission is conditioned in the third or fourth (overdrive) gear position. Upon presence of the hydraulic pressure signal in this fluid line, the 3-4 shift valve is held or locked at the low gear position. Thus, the 3-4 shift valve is held at a low gear position thereof owing to the presence of the hydraulic pressure in the fluid line leading from the 2-3 shift valve during forward running with the first and second gear position during the forward drive range or during reverse drive range. If the solenoids of both of these electro-hydraulic transducers are inadvertently deenergized when the automatic transmission has the reverse drive range, the 2-3 shift valve is urged against the spring to the above-mentioned position wherein the predetermined fluid line is drained, allowing the 3-4 shift valve to move to the high gear position thereof owing to the solenoid hydraulic pressure. As a result, the auxiliary gearing tending to rotate as a unit is anchored by the one-way clutch, and it is interlocked.
An object of the present invention is to provide an automatic transmission including an auxiliary gearing which has a shift control for the auxiliary gearing free from the above-mentioned problem.
A specific object of the present invention is to provide a shift control for the auxiliary gearing of the automatic transmission which will hold the auxiliary gearing in the low gear position thereof when the automatic transmission has a reverse drive range (R range).
Another and further specific object of the present invention is to provide a shift control whereby the auxiliary gearing is shifted to the high gear position thereof when the automatic transmission has a forward drive range if the associated solenoid or solenoids are inadvertently deenergized.